The present invention relates to a method for attenuating interference in a positioning receiver of an electronic device, in which a signal transmitted by satellites of a positioning system is received through a second antenna, and which electronic device further comprises at least a mobile station and a first antenna to be used at least as a transmitting antenna for the mobile station. The invention also relates to an electronic device, which comprises at least a mobile station, a first antenna to be used at least as the transmitting antenna of the mobile station, a positioning receiver, a second antenna for receiving a signal transmitted by satellites of a positioning system to the positioning receiver, and means for attenuating interference.
Devices are known in which a positioning receiver is used for determining the location of the device. Such positioning receivers are primarily based on satellite-based positioning systems. In such satellite-based positioning systems the positioning receiver attempts to receive a signal transmitted by satellites and containing phase-modulated information, inter alia the orbital parameters of satellites. In practice, however, the signal strength in the positioning receiver may be so attenuated, particularly indoors, that signal detection is difficult, and positioning cannot always be performed.
One known positioning system is the GPS system (Global Positioning System), which comprises more than 30 satellites, of which usually a maximum of 12 are simultaneously within the sight of a receiver. These satellites transmit e.g. Ephemeris data of the satellite, as well as time data of the satellite. The receiver used in the positioning normally determines its position by computing the propagation time of a plurality of signals transmitted substantially simultaneously from several satellites belonging to the positioning system to the receiver. For positioning, the receiver must typically receive signals of at least four satellites within sight, to be able to compute the position.
Each operating satellite of the GPS system transmits a so-called L1 signal at the carrier frequency of 1575.42 MHz. This frequency is also indicated with 154 f0, where f0=10.23 MHz. Furthermore, the satellites transmit another ranging signal at a carrier frequency of 1227.6 MHz called L2, i.e. 120 f0. The carrier frequency of the L1 signal is further modulated by navigation information at a bit rate of 50 bit/s. The navigation information comprises information about the xe2x80x9chealthxe2x80x9d and orbit of the satellite, parameters related to the local clock of the satellite, etc. In satellites of the GPS system, e.g. so-called atomic clocks are used as the local clock.
There are devices under development that have both a positioning receiver and means for performing mobile station functions, wherein an object of the device is both to perform positioning and to use the device as a mobile station. Typically, a frequency range at the frequency of 900 MHz, 1800 MHz and/or 1900 MHz has been allocated for the use of mobile communication systems. Some devices can be connected to two or even three mobile communication systems operating at different frequencies.
Particularly the transmitter of the mobile station generates strong signals at the frequency range of the mobile communication system and also at harmonic frequencies. This interference signal is broadband to such a degree that it raises the noise level of the frequency range used in the positioning. For example a GSM transmission produces a primary transmission signal fGSM and an interfering signal fGSMnoise. Broadband noise caused by a GSM transmission is illustrated in FIG. 1, where it can be seen that the broadband noise produced at GSM 1800 is most significant to the GPS band. Other noise that is disturbing in view of the GPS positioning receiver includes for example such interfering frequencies caused by a GSM transmission that arrive at a GPS band. These noise signals are connected particularly via the antenna of the mobile station to the antenna of the positioning receiver, but some of them can also be connected inside the device.
Therefore, the reception of signals from satellites in a positioning system is most difficult during the transmission of the mobile station. Even if an aim has been to implement the structure of the device in a manner that such interference signals would not spread into the surrounding space, the device may also, to some extent, radiate interference signals into its close vicinity. It is also problematic to attenuate such interference signals inside the device itself. In this case, in a prior art device the interference radiation enters the positioning receiver, wherein the positioning receiver is not necessarily capable of detecting weak signals transmitted by satellites, and the positioning fails. Sufficient attenuation of interference radiation is difficult with different radiation shields and earthing arrangements inside the device itself, and in many cases this would require increasing the size of the device. In some prior art devices an aim has been to solve this problem in a manner that the positioning receiver is not in operative state when the mobile station is operating. This causes problems in practical use, because the user must turn off the mobile station in order to perform positioning. In a corresponding manner, the user must turn off the positioning receiver when he/she switches on the mobile station. Since particularly weak signal strength causes that the positioning often takes a long time, even several minutes, usability of such a device is not particularly good.
In some cases, the positioning of a mobile station can be performed by using the mobile station and base stations of a mobile communication network, in case the service provider of the mobile communication system has introduced this kind of service. Thus, this system is dependent on the service provider and therefore not a general solution to the positioning problem. Another aspect that could be mentioned about such a solution is that even if the service were available in a mobile communication network of a mobile communication service provider, the service provider may charge for the use of the service. The user does not necessarily want to pay for using such a system.
In some countries it has been or will be made compulsory to provide the mobile station with the function whereby it can be positioned, particularly during an emergency call. In this case the mobile station must not be switched off while performing positioning. In practice, this means that when placing an emergency call, the positioning in a prior art system should be performed by using a mobile communication network, which, as has been mentioned, is not always possible.
An aim of the present invention is to implement an improved method for reducing interference occurring in an electronic device, in a positioning receiver located in the electronic device and used in the positioning. The invention is based on the idea that a compensation signal is formed of signals generated in the electronic device and causing interference in the positioning receiver, such as a signal generated by a transmitter, which compensation signal is transmitted to the positioning receiver as substantially reverse phased relative to the signal formed by the transmitter in a manner that the compensation signal attenuates the effect of the signal formed by the transmitter in the positioning receiver to a significant extent. To be exact, the method according to the present invention is primarily characterized in that in the method, the strength of at least one interference signal formed in the electronic device is determined, a compensation signal corresponding to the strength of the determined interference signal is formed, and said compensation signal is combined with a signal received through a second antenna, which combined signal is directed to the positioning receiver. The electronic device according to the present invention is primarily characterized in that means for attenuating interference comprise means for determining at least the strength of one interference signal formed in the electronic device, means for forming a compensation signal corresponding to the strength of the determined interference signal, and means for forming a combined signal by combining said compensation signal with a signal received through the second antenna, and means for directing the combined signal to a positioning receiver.
The present invention shows remarkable advantages compared to solutions of prior art. By using a method according to the invention, the influence of the radiation caused by the transmitter to the operation of the positioning receiver can be attenuated to a significant degree, wherein the positioning receiver can be kept operative also when the mobile station is not operative. In this case, the user can for example during a call examine his/her location and, if necessary, keep track on the changes in his/her location. Thus, the user can transmit his/her positioning data through a telephone call to the person with whom he/she is speaking, who can e.g. give driving directions to a certain location. In addition, from the electronic device in accordance with an advantageous embodiment of the invention, positioning data can be automatically transmitted e.g. during an emergency call, wherein it is easier and faster to direct help to the correct location than when prior art solutions are used.